1. Field of the Invention
The present invention relates to an ion conductive material and an electrochemical device comprising the same. In particular, the present invention relates to an ion conductive material which is excellent in a high voltage stability such as an electrolyte, and an electrochemical device such as a cell, an electrolytic system, an electrochemical sensor, and so on.
2. Description of the Related Art
Since an electrochemical device which is represented by a manganese dioxide-lithium cell can generate a high voltage of about 3 V and has a high energy density, the demand therefor is increasing. Recently, a lithium ion secondary cell using LiCoO.sub.2 and carbon has been developed, and the demand therefor has been greatly increased, since it can generate a high voltage of about 3.6 V.
Hitherto, as an electrolyte of such type of the cell, a perchlorate compound such as LiCIO.sub.4 is used, but it is less used recently because of the problem of safety. It has been proposed to improve the safety and reliability of the device by dispersing various lithium salts including above LiCIO.sub.4 in a polymer such as polyethylene oxide (PEO) to form a solid ion conductive material. However, the formation of the solid ion conductive material will decrease the ion conductivity considerably.
It has been proposed to use an organic lithium salt such as LiB(C.sub.6 H.sub.5).sub.4, LiCF.sub.3 SO.sub.3, LiCF.sub.3 CO.sub.2, LiN(CF.sub.3 SO.sub.2).sub.2, LiC(CF.sub.3 SO.sub.2).sub.3, etc. as an electrolyte instead of LiCIO.sub.4.
But, above LiB(C.sub.6 H.sub.5).sub.4 has problems that it has insufficient high voltage stability, and its storage stability is low depending on a kind of a solvent. In addition, an electrolytic solution comprising a solution of LiB(C.sub.6 H.sub.5).sub.4 dissolved in an organic solvent (a liquid ion conductive material) is discolored during storage, or some kind of the solvent is polymerized. As a result, the performance of the cell comprising such electrolytic solution decreases during the storage.
The organic lithium salts such as LiCF.sub.3 SO.sub.3, LiCF.sub.3 CO.sub.2, LiN(CF.sub.3 SO.sub.2).sub.2, LiC(CF.sub.3 SO.sub.2).sub.3, etc. have better safety than LiCIO.sub.4, but have problems that oxidation stability is poor, stability at a high voltage is low, and ion conductivity is low, Additionally, they may not be used together with some materials of electrode collectors, and they are expensive.